Internet Engineering Task Force (IETF) J. Rosenberg
Request for Comments: 5245 jdrosen.net
Obsoletes: 4091, 4092 April 2010
Category: Standards Track
ISSN: 2070-1721
Interactive Connectivity Establishment (ICE):
A Protocol for Network Address Translator (NAT) Traversal for
Offer/Answer Protocols
Abstract
This document describes a protocol for Network Address Translator
(NAT) traversal for UDP-based multimedia sessions established with
the offer/answer model. This protocol is called Interactive
Connectivity Establishment (ICE). ICE makes use of the Session
Traversal Utilities for NAT (STUN) protocol and its extension,
Traversal Using Relay NAT (TURN). ICE can be used by any protocol
utilizing the offer/answer model, such as the Session Initiation
Protocol (SIP).
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5245.

implementors are making assumptions about the topologies of the
networks in which their solutions will be deployed. This introduces
complexity and brittleness into the system. What is needed is a
single solution that is flexible enough to work well in all
situations.
This specification defines Interactive Connectivity Establishment
(ICE) as a technique for NAT traversal for UDP-based media streams
(though ICE can be extended to handle other transport protocols, such
as TCP [ICE-TCP]) established by the offer/answer model. ICE is an
extension to the offer/answer model, and works by including a
multiplicity of IP addresses and ports in SDP offers and answers,
which are then tested for connectivity by peer-to-peer connectivity
checks. The IP addresses and ports included in the SDP and the
connectivity checks are performed using the revised STUN
specification [RFC5389], now renamed to Session Traversal Utilities
for NAT. The new name and new specification reflect its new role as
a tool that is used with other NAT traversal techniques (namely ICE)
rather than a standalone NAT traversal solution, as the original STUN
specification was. ICE also makes use of Traversal Using Relays
around NAT (TURN) [RFC5766], an extension to STUN. Because ICE
exchanges a multiplicity of IP addresses and ports for each media
stream, it also allows for address selection for multihomed and dual-
stack hosts, and for this reason it deprecates RFC 4091 [RFC4091] and
[RFC4092].
2. Overview of ICE
In a typical ICE deployment, we have two endpoints (known as AGENTS
in RFC 3264 terminology) that want to communicate. They are able to
communicate indirectly via some signaling protocol (such as SIP), by
which they can perform an offer/answer exchange of SDP [RFC3264]
messages. Note that ICE is not intended for NAT traversal for SIP,
which is assumed to be provided via another mechanism [RFC5626]. At
the beginning of the ICE process, the agents are ignorant of their
own topologies. In particular, they might or might not be behind a
NAT (or multiple tiers of NATs). ICE allows the agents to discover
enough information about their topologies to potentially find one or
more paths by which they can communicate.
Figure 1 shows a typical environment for ICE deployment. The two
endpoints are labelled L and R (for left and right, which helps
visualize call flows). Both L and R are behind their own respective
NATs though they may not be aware of it. The type of NAT and its
properties are also unknown. Agents L and R are capable of engaging
in an offer/answer exchange by which they can exchange SDP messages,
whose purpose is to set up a media session between L and R.
Typically, this exchange will occur through a SIP server.

practice, however, many combinations will not work. For instance, if
L and R are both behind NATs, their directly attached interface
addresses are unlikely to be able to communicate directly (this is
why ICE is needed, after all!). The purpose of ICE is to discover
which pairs of addresses will work. The way that ICE does this is to
systematically try all possible pairs (in a carefully sorted order)
until it finds one or more that work.
2.1. Gathering Candidate Addresses
In order to execute ICE, an agent has to identify all of its address
candidates. A CANDIDATE is a transport address -- a combination of
IP address and port for a particular transport protocol (with only
UDP specified here). This document defines three types of
candidates, some derived from physical or logical network interfaces,
others discoverable via STUN and TURN. Naturally, one viable
candidate is a transport address obtained directly from a local
interface. Such a candidate is called a HOST CANDIDATE. The local
interface could be ethernet or WiFi, or it could be one that is
obtained through a tunnel mechanism, such as a Virtual Private
Network (VPN) or Mobile IP (MIP). In all cases, such a network
interface appears to the agent as a local interface from which ports
(and thus candidates) can be allocated.
If an agent is multihomed, it obtains a candidate from each IP
address. Depending on the location of the PEER (the other agent in
the session) on the IP network relative to the agent, the agent may
be reachable by the peer through one or more of those IP addresses.
Consider, for example, an agent that has a local IP address on a
private net 10 network (I1), and a second connected to the public
Internet (I2). A candidate from I1 will be directly reachable when
communicating with a peer on the same private net 10 network, while a
candidate from I2 will be directly reachable when communicating with
a peer on the public Internet. Rather than trying to guess which IP
address will work prior to sending an offer, the offering agent
includes both candidates in its offer.
Next, the agent uses STUN or TURN to obtain additional candidates.
These come in two flavors: translated addresses on the public side of
a NAT (SERVER REFLEXIVE CANDIDATES) and addresses on TURN servers
(RELAYED CANDIDATES). When TURN servers are utilized, both types of
candidates are obtained from the TURN server. If only STUN servers
are utilized, only server reflexive candidates are obtained from
them. The relationship of these candidates to the host candidate is
shown in Figure 2. In this figure, both types of candidates are
discovered using TURN. In the figure, the notation X:x means IP
address X and UDP port x.

To Internet
|
|
| /------------ Relayed
Y:y | / Address
+--------+
| |
| TURN |
| Server |
| |
+--------+
|
|
| /------------ Server
X1':x1'|/ Reflexive
+------------+ Address
| NAT |
+------------+
|
| /------------ Local
X:x |/ Address
+--------+
| |
| Agent |
| |
+--------+
Figure 2: Candidate Relationships
When the agent sends the TURN Allocate request from IP address and
port X:x, the NAT (assuming there is one) will create a binding
X1':x1', mapping this server reflexive candidate to the host
candidate X:x. Outgoing packets sent from the host candidate will be
translated by the NAT to the server reflexive candidate. Incoming
packets sent to the server reflexive candidate will be translated by
the NAT to the host candidate and forwarded to the agent. We call
the host candidate associated with a given server reflexive candidate
the BASE.
Note: "Base" refers to the address an agent sends from for a
particular candidate. Thus, as a degenerate case host candidates
also have a base, but it's the same as the host candidate.
When there are multiple NATs between the agent and the TURN server,
the TURN request will create a binding on each NAT, but only the
outermost server reflexive candidate (the one nearest the TURN

server) will be discovered by the agent. If the agent is not behind
a NAT, then the base candidate will be the same as the server
reflexive candidate and the server reflexive candidate is redundant
and will be eliminated.
The Allocate request then arrives at the TURN server. The TURN
server allocates a port y from its local IP address Y, and generates
an Allocate response, informing the agent of this relayed candidate.
The TURN server also informs the agent of the server reflexive
candidate, X1':x1' by copying the source transport address of the
Allocate request into the Allocate response. The TURN server acts as
a packet relay, forwarding traffic between L and R. In order to send
traffic to L, R sends traffic to the TURN server at Y:y, and the TURN
server forwards that to X1':x1', which passes through the NAT where
it is mapped to X:x and delivered to L.
When only STUN servers are utilized, the agent sends a STUN Binding
request [RFC5389] to its STUN server. The STUN server will inform
the agent of the server reflexive candidate X1':x1' by copying the
source transport address of the Binding request into the Binding
response.
2.2. Connectivity Checks
Once L has gathered all of its candidates, it orders them in highest
to lowest priority and sends them to R over the signaling channel.
The candidates are carried in attributes in the SDP offer. When R
receives the offer, it performs the same gathering process and
responds with its own list of candidates. At the end of this
process, each agent has a complete list of both its candidates and
its peer's candidates. It pairs them up, resulting in CANDIDATE
PAIRS. To see which pairs work, each agent schedules a series of
CHECKS. Each check is a STUN request/response transaction that the
client will perform on a particular candidate pair by sending a STUN
request from the local candidate to the remote candidate.
The basic principle of the connectivity checks is simple:
1. Sort the candidate pairs in priority order.
2. Send checks on each candidate pair in priority order.
3. Acknowledge checks received from the other agent.

With both agents performing a check on a candidate pair, the result
is a 4-way handshake:
L R
- -
STUN request -> \ L's
<- STUN response / check
<- STUN request \ R's
STUN response -> / check
Figure 3: Basic Connectivity Check
It is important to note that the STUN requests are sent to and from
the exact same IP addresses and ports that will be used for media
(e.g., RTP and RTCP). Consequently, agents demultiplex STUN and RTP/
RTCP using contents of the packets, rather than the port on which
they are received. Fortunately, this demultiplexing is easy to do,
especially for RTP and RTCP.
Because a STUN Binding request is used for the connectivity check,
the STUN Binding response will contain the agent's translated
transport address on the public side of any NATs between the agent
and its peer. If this transport address is different from other
candidates the agent already learned, it represents a new candidate,
called a PEER REFLEXIVE CANDIDATE, which then gets tested by ICE just
the same as any other candidate.
As an optimization, as soon as R gets L's check message, R schedules
a connectivity check message to be sent to L on the same candidate
pair. This accelerates the process of finding a valid candidate, and
is called a TRIGGERED CHECK.
At the end of this handshake, both L and R know that they can send
(and receive) messages end-to-end in both directions.
2.3. Sorting Candidates
Because the algorithm above searches all candidate pairs, if a
working pair exists it will eventually find it no matter what order
the candidates are tried in. In order to produce faster (and better)
results, the candidates are sorted in a specified order. The
resulting list of sorted candidate pairs is called the CHECK LIST.
The algorithm is described in Section 4.1.2 but follows two general
principles:
o Each agent gives its candidates a numeric priority, which is sent
along with the candidate to the peer.

o The local and remote priorities are combined so that each agent
has the same ordering for the candidate pairs.
The second property is important for getting ICE to work when there
are NATs in front of L and R. Frequently, NATs will not allow
packets in from a host until the agent behind the NAT has sent a
packet towards that host. Consequently, ICE checks in each direction
will not succeed until both sides have sent a check through their
respective NATs.
The agent works through this check list by sending a STUN request for
the next candidate pair on the list periodically. These are called
ORDINARY CHECKS.
In general, the priority algorithm is designed so that candidates of
similar type get similar priorities and so that more direct routes
(that is, through fewer media relays and through fewer NATs) are
preferred over indirect ones (ones with more media relays and more
NATs). Within those guidelines, however, agents have a fair amount
of discretion about how to tune their algorithms.
2.4. Frozen Candidates
The previous description only addresses the case where the agents
wish to establish a media session with one COMPONENT (a piece of a
media stream requiring a single transport address; a media stream may
require multiple components, each of which has to work for the media
stream as a whole to be work). Typically (e.g., with RTP and RTCP),
the agents actually need to establish connectivity for more than one
flow.
The network properties are likely to be very similar for each
component (especially because RTP and RTCP are sent and received from
the same IP address). It is usually possible to leverage information
from one media component in order to determine the best candidates
for another. ICE does this with a mechanism called "frozen
candidates".
Each candidate is associated with a property called its FOUNDATION.
Two candidates have the same foundation when they are "similar" -- of
the same type and obtained from the same host candidate and STUN
server using the same protocol. Otherwise, their foundation is
different. A candidate pair has a foundation too, which is just the
concatenation of the foundations of its two candidates. Initially,
only the candidate pairs with unique foundations are tested. The
other candidate pairs are marked "frozen". When the connectivity
checks for a candidate pair succeed, the other candidate pairs with

the same foundation are unfrozen. This avoids repeated checking of
components that are superficially more attractive but in fact are
likely to fail.
While we've described "frozen" here as a separate mechanism for
expository purposes, in fact it is an integral part of ICE and the
ICE prioritization algorithm automatically ensures that the right
candidates are unfrozen and checked in the right order.
2.5. Security for Checks
Because ICE is used to discover which addresses can be used to send
media between two agents, it is important to ensure that the process
cannot be hijacked to send media to the wrong location. Each STUN
connectivity check is covered by a message authentication code (MAC)
computed using a key exchanged in the signaling channel. This MAC
provides message integrity and data origin authentication, thus
stopping an attacker from forging or modifying connectivity check
messages. Furthermore, if the SIP [RFC3261] caller is using ICE, and
their call forks, the ICE exchanges happen independently with each
forked recipient. In such a case, the keys exchanged in the
signaling help associate each ICE exchange with each forked
recipient.
2.6. Concluding ICE
ICE checks are performed in a specific sequence, so that high-
priority candidate pairs are checked first, followed by lower-
priority ones. One way to conclude ICE is to declare victory as soon
as a check for each component of each media stream completes
successfully. Indeed, this is a reasonable algorithm, and details
for it are provided below. However, it is possible that a packet
loss will cause a higher-priority check to take longer to complete.
In that case, allowing ICE to run a little longer might produce
better results. More fundamentally, however, the prioritization
defined by this specification may not yield "optimal" results. As an
example, if the aim is to select low-latency media paths, usage of a
relay is a hint that latencies may be higher, but it is nothing more
than a hint. An actual round-trip time (RTT) measurement could be
made, and it might demonstrate that a pair with lower priority is
actually better than one with higher priority.
Consequently, ICE assigns one of the agents in the role of the
CONTROLLING AGENT, and the other of the CONTROLLED AGENT. The
controlling agent gets to nominate which candidate pairs will get
used for media amongst the ones that are valid. It can do this in
one of two ways -- using REGULAR NOMINATION or AGGRESSIVE NOMINATION.

With regular nomination, the controlling agent lets the checks
continue until at least one valid candidate pair for each media
stream is found. Then, it picks amongst those that are valid, and
sends a second STUN request on its NOMINATED candidate pair, but this
time with a flag set to tell the peer that this pair has been
nominated for use. This is shown in Figure 4.
L R
- -
STUN request -> \ L's
<- STUN response / check
<- STUN request \ R's
STUN response -> / check
STUN request + flag -> \ L's
<- STUN response / check
Figure 4: Regular Nomination
Once the STUN transaction with the flag completes, both sides cancel
any future checks for that media stream. ICE will now send media
using this pair. The pair an ICE agent is using for media is called
the SELECTED PAIR.
In aggressive nomination, the controlling agent puts the flag in
every STUN request it sends. This way, once the first check
succeeds, ICE processing is complete for that media stream and the
controlling agent doesn't have to send a second STUN request. The
selected pair will be the highest-priority valid pair whose check
succeeded. Aggressive nomination is faster than regular nomination,
but gives less flexibility. Aggressive nomination is shown in
Figure 5.
L R
- -
STUN request + flag -> \ L's
<- STUN response / check
<- STUN request \ R's
STUN response -> / check
Figure 5: Aggressive Nomination
Once all of the media streams are completed, the controlling endpoint
sends an updated offer if the candidates in the m and c lines for the
media stream (called the DEFAULT CANDIDATES) don't match ICE's
SELECTED CANDIDATES.

Once ICE is concluded, it can be restarted at any time for one or all
of the media streams by either agent. This is done by sending an
updated offer indicating a restart.
2.7. Lite Implementations
In order for ICE to be used in a call, both agents need to support
it. However, certain agents will always be connected to the public
Internet and have a public IP address at which it can receive packets
from any correspondent. To make it easier for these devices to
support ICE, ICE defines a special type of implementation called LITE
(in contrast to the normal FULL implementation). A lite
implementation doesn't gather candidates; it includes only host
candidates for any media stream. Lite agents do not generate
connectivity checks or run the state machines, though they need to be
able to respond to connectivity checks. When a lite implementation
connects with a full implementation, the full agent takes the role of
the controlling agent, and the lite agent takes on the controlled
role. When two lite implementations connect, no checks are sent.
For guidance on when a lite implementation is appropriate, see the
discussion in Appendix A.
It is important to note that the lite implementation was added to
this specification to provide a stepping stone to full
implementation. Even for devices that are always connected to the
public Internet, a full implementation is preferable if achievable.
3. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Readers should be familiar with the terminology defined in the offer/
answer model [RFC3264], STUN [RFC5389], and NAT Behavioral
requirements for UDP [RFC4787].
This specification makes use of the following additional terminology:
Agent: As defined in RFC 3264, an agent is the protocol
implementation involved in the offer/answer exchange. There are
two agents involved in an offer/answer exchange.

Peer: From the perspective of one of the agents in a session, its
peer is the other agent. Specifically, from the perspective of
the offerer, the peer is the answerer. From the perspective of
the answerer, the peer is the offerer.
Transport Address: The combination of an IP address and transport
protocol (such as UDP or TCP) port.
Candidate: A transport address that is a potential point of contact
for receipt of media. Candidates also have properties -- their
type (server reflexive, relayed or host), priority, foundation,
and base.
Component: A component is a piece of a media stream requiring a
single transport address; a media stream may require multiple
components, each of which has to work for the media stream as a
whole to work. For media streams based on RTP, there are two
components per media stream -- one for RTP, and one for RTCP.
Host Candidate: A candidate obtained by binding to a specific port
from an IP address on the host. This includes IP addresses on
physical interfaces and logical ones, such as ones obtained
through Virtual Private Networks (VPNs) and Realm Specific IP
(RSIP) [RFC3102] (which lives at the operating system level).
Server Reflexive Candidate: A candidate whose IP address and port
are a binding allocated by a NAT for an agent when it sent a
packet through the NAT to a server. Server reflexive candidates
can be learned by STUN servers using the Binding request, or TURN
servers, which provides both a relayed and server reflexive
candidate.
Peer Reflexive Candidate: A candidate whose IP address and port are
a binding allocated by a NAT for an agent when it sent a STUN
Binding request through the NAT to its peer.
Relayed Candidate: A candidate obtained by sending a TURN Allocate
request from a host candidate to a TURN server. The relayed
candidate is resident on the TURN server, and the TURN server
relays packets back towards the agent.
Base: The base of a server reflexive candidate is the host candidate
from which it was derived. A host candidate is also said to have
a base, equal to that candidate itself. Similarly, the base of a
relayed candidate is that candidate itself.

Foundation: An arbitrary string that is the same for two candidates
that have the same type, base IP address, protocol (UDP, TCP,
etc.), and STUN or TURN server. If any of these are different,
then the foundation will be different. Two candidate pairs with
the same foundation pairs are likely to have similar network
characteristics. Foundations are used in the frozen algorithm.
Local Candidate: A candidate that an agent has obtained and included
in an offer or answer it sent.
Remote Candidate: A candidate that an agent received in an offer or
answer from its peer.
Default Destination/Candidate: The default destination for a
component of a media stream is the transport address that would be
used by an agent that is not ICE aware. For the RTP component,
the default IP address is in the c line of the SDP, and the port
is in the m line. For the RTCP component, it is in the rtcp
attribute when present, and when not present, the IP address is in
the c line and 1 plus the port is in the m line. A default
candidate for a component is one whose transport address matches
the default destination for that component.
Candidate Pair: A pairing containing a local candidate and a remote
candidate.
Check, Connectivity Check, STUN Check: A STUN Binding request
transaction for the purposes of verifying connectivity. A check
is sent from the local candidate to the remote candidate of a
candidate pair.
Check List: An ordered set of candidate pairs that an agent will use
to generate checks.
Ordinary Check: A connectivity check generated by an agent as a
consequence of a timer that fires periodically, instructing it to
send a check.
Triggered Check: A connectivity check generated as a consequence of
the receipt of a connectivity check from the peer.
Valid List: An ordered set of candidate pairs for a media stream
that have been validated by a successful STUN transaction.
Full: An ICE implementation that performs the complete set of
functionality defined by this specification.

Lite: An ICE implementation that omits certain functions,
implementing only as much as is necessary for a peer
implementation that is full to gain the benefits of ICE. Lite
implementations do not maintain any of the state machines and do
not generate connectivity checks.
Controlling Agent: The ICE agent that is responsible for selecting
the final choice of candidate pairs and signaling them through
STUN and an updated offer, if needed. In any session, one agent
is always controlling. The other is the controlled agent.
Controlled Agent: An ICE agent that waits for the controlling agent
to select the final choice of candidate pairs.
Regular Nomination: The process of picking a valid candidate pair
for media traffic by validating the pair with one STUN request,
and then picking it by sending a second STUN request with a flag
indicating its nomination.
Aggressive Nomination: The process of picking a valid candidate pair
for media traffic by including a flag in every STUN request, such
that the first one to produce a valid candidate pair is used for
media.
Nominated: If a valid candidate pair has its nominated flag set, it
means that it may be selected by ICE for sending and receiving
media.
Selected Pair, Selected Candidate: The candidate pair selected by
ICE for sending and receiving media is called the selected pair,
and each of its candidates is called the selected candidate.